U.S. Pat. No. 4,632,852 to Akahoshi et al. describes an electroless copper plating process for additive printed wiring boards. The copper deposits obtained are described as having tensile strengths of to 442 Pa and elongation on 30 .mu.m thick test strips ranging from 3.8% to 11.1%. The electroless copper plating solutions are operated at temperatures of 40.degree.-80.degree. C., a sodium hydroxide concentration which will give a pH of 12.8 (measured at 25.degree. C.), and are characterized by the stabilizers comprising 1-100mg/L of silicon or germanium in the solution and at least 0.01 L/min per liter of injected air per liter of plating solution.
U.S. Pat. No. 4,865,888 to Akahoshi et al. describes an improvement in the process of U.S. Pat. No. 4,632,852 wherein the air is injected in fine bubbles with a bubble diameter of 1 mm or less.
The copper deposits formed by the processes of U.S. Pat. Nos. 4,632,852 and 4,865,888 have a rough surface. Because many additive printed wiring applications require a smooth surface, the surface roughness of the copper limits its use. The roughness of the deposits is demonstrated in U.S. Pat. No. 4,970,107 to Akahoshi et al. which describes a process wherein the electroless copper deposition bath described in U.S. Pat. No. 4,632,852 is employed to plate over a smooth copper surface with an electroless copper deposit to a thickness of 5 .mu.m. The surface thus produced has knife-shaped projections for improving the bond when the copper surface is laminated in a multilayer printed wiring board. An additive selected from oxoacids of silicon, germanium, vanadium or carbon is employed in the electroless bath to promote formation of the knife-shaped projections.
U.S. Pat. No. 3,959,531 to Schneble et al. indicates that iron in electroless copper deposition solutions should be below 25 mg/l preferably below 10 mg/l in order to reduce extraneous copper deposits and avoid spontaneous decomposition of the plating solution. There is no teaching in U.S. Pat. No. 3,959,531 that trace quantities of codeposited iron affect the physical properties of the electrolessly deposited copper. The control of the iron content was important only for the stability of the electroless copper bath, not for the quality of the copper deposit.
U.S. Pat. No. 3,485,643 to Zeblisky et al. describes hexacyanoferrates as stabilizers in electroless copper deposition solutions. U.S. Pat. No. 4,650,691 to Kinoshita etal. reports that the hexacyanoferrates decompose in the plating bath and that the decomposition products inhibit further electroless copper plating and form precipitates in the bath. Kinoshita et al. indicate that the use of triethanolamine at one to three times the molar concentration of the hexacyanoferrate in the electroless bath will prevent destruction of the bath by inhibition of the plating reaction and the formation of precipitates. The us of triethanolamine is taught for preventing iron precipitation in the plating solution, and not for improving the quality of the copper deposit.
U.S. Pat. No. 3,310,430 to Schneble et al. describes electroless copper baths containing a vanadium compound as a hydrogen inclusion retarding agent Copper is deposited as thin foils (5-12 .mu.m thick) with sufficient ductility to permit folding the foils in half, creasing them and unfolding the foils up to 5 times before the foil breaks at the crease.
U.S. Pat. No. 4,563,217 to Kikuchi et al. describes electroless copper baths comprising a cationic wetting agent and an inorganic compound containing silicon, germanium or vanadium to improve the stability of the plating bath and tensile strength and percent elongation of the deposited copper. The copper deposits reported had tensile strengths of 324-657 MPa and percent elongation for 50 .mu.m thick foils of 2-7%. If these percent elongations are normalized to copper foil 30 .mu.m thick, the range would be 2-5%.